A large quantity of flue gases are produced from fossil fuel combustion and discharged into atmosphere. In addition to sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, a small quantity of harmful organic substances and heavy metal compounds, flue gases contain a large quantity of dusts, and there are tiny hydrophilic and lipophilic particles included in these dusts, wherein said tiny hydrophilic and lipophilic particles are mainly comprised of calcium salt particles, aluminum salt particles, magnesium salt particles, titanium salt particles, iron salt particles, lead salt particles, zinc salt particles, cobalt salt particles, rare earth element particles, radioactive element particles and particles of other harmful elements, as well as mineral particles such as silica particles, mullite particles, silicate particles and phosphate particles. These particles are discharged together with the flue gases into atmosphere, and heavy metal compounds, sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, dioxins, polycyclic aromatic hydrocarbons, other harmful organic substances, bacteria, and the like are readily adsorbed on the surface of these particles, causing a significant increase in the content of atmospheric suspended particulates (i.e., PM100, PM10, PM2.5, etc.), resulting in the phenomena of haze and atmospheric photochemical reactions, and causing serious environmental pollution.
Currently, the vast majority of waste gas desulfurization methods adopt limestone or lime wet methods for desulfurization.
In the limestone wet desulfurization method, calcium carbonate stones from mines are broken by a crusher, ground into fine powders with a mesh number greater than 325 by a ball mill, and then formulated into a slurry containing 10%-15% calcium carbonate powder. In a desulfurization tower, the calcium carbonate slurry contacts with the flue gas, and sulfur dioxide in the gas reacts with calcium carbonate in the slurry to produce calcium sulfite. In an air forced oxidation layer of the desulfurization tower, the calcium sulfite slurry is oxidized into calcium sulfate. Calcium sulfate, which also contains some calcium sulfite, is separated, and calcium sulfite will decompose and release sulfur dioxide, resulting in secondary pollution. In particular, the tiny hydrophilic and lipophilic particles contained in the ground calcium carbonate slurry are brought out by the flue gas and discharged into atmosphere, and heavy metal compounds, sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, dioxins, polycyclic aromatic hydrocarbons, other harmful organic substances, bacteria, and the like are readily adsorbed on the surface of these particles, causing a significant increase in the content of atmospheric suspended particulates (i.e., PM100, PM10, PM2.5, etc.), resulting in the phenomena of haze and atmospheric photochemical reactions, and causing serious environmental pollution. Wherein, said tiny hydrophilic and lipophilic particles are mainly comprised of calcium salt particles, aluminum salt particles, magnesium salt particles, titanium salt particles, iron salt particles, lead salt particles, zinc salt particles, cobalt salt particles, rare earth element particles, radioactive element particles and particles of other harmful elements, as well as mineral particles such as silica particles, mullite particles, silicate particles and phosphate particles, and the like.
In the lime wet desulfurization process, calcined calcium oxide is utilized to react with water, producing an aqueous emulsion of calcium hydroxide, which is formulated into a slurry containing 10%-15% calcium hydroxide. In a desulfurization tower, the calcium hydroxide slurry contacts with the flue gas, and sulfur dioxide in the gas reacts with calcium hydroxide in the slurry to produce calcium sulfite, thus the calcium hydroxide slurry is converted into a calcium sulfite slurry. In an air forced oxidation layer of the desulfurization tower, the calcium sulfite slurry is oxidized into calcium sulfate, thus the calcium sulfite slurry is converted into a calcium sulfate slurry. The calcium sulfate slurry flows out of the desulfurization tower and enters a separator for separation of calcium sulfate from the slurry. Separated calcium sulfate also contains some solid wastes such as calcium sulfite, calcium carbonate and unreacted calcium hydroxide, and calcium sulfite will decompose and release sulfur dioxide, resulting in pollution transfer and secondary pollution. Besides, in the calcination process for producing calcium oxide, a large quantity of coals are consumed, which also causes serious pollution. At the same time, since tiny hydrophilic and lipophilic particles contained in the calcium hydroxide slurry are brought out by the flue gas and discharged into atmosphere, and heavy metal compounds, sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, dioxins, polycyclic aromatic hydrocarbons, other harmful organic substances, bacteria, and the like are readily adsorbed on the surface of these particles, causing a significant increase in the content of atmospheric suspended particulate (i.e., PM100, PM10, PM2.5, etc.), resulting in the phenomena of haze and atmospheric photochemical reactions, and causing serious environmental pollution. Wherein, said tiny hydrophilic and lipophilic particles are mainly comprised of calcium salt particles, aluminum salt particles, magnesium salt particles, titanium salt particles, iron salt particles, lead salt particles, zinc salt particles, cobalt salt particles, rare earth element particles, radioactive element particles and particles of other harmful elements, as well as mineral particles such as silica particles, mullite particles, silicate particles and phosphate particles, and the like.
After being desulfurized by limestone or lime wet method, waste gases contain a large quantity of tiny particles that cannot be removed by conventional dedusting methods (such as bag-type dedusting method). If the waste gases are discharged directly into atmosphere without being further dedusted, phenomena of haze and atmospheric photochemical reactions will be strengthened significantly. In particular, organic substances that are extremely hazardous to human and creatures cannot be removed from the discharged waste gases, and discharging these organic substances into atmosphere will seriously affect and endanger human health.
Conventional waste gas dedusting methods include electrostatic dedusting method, cyclone dedusting method, bag-type dedusting method, water-dampness type dedusting method, and the like.
The basic principle of electrostatic dedusting method is that, the dust-containing gas is electrically separated while passing through a high-voltage electrostatic field, whereby dust particles combine with negative ions and become negatively charged, then run towards anode surface, being discharged and deposited. In a strong electric field, air molecules are ionized into positive ions and electrons, and electrons encounter the dust particles while running towards the positive electrode, so that the dust particles are negatively charged, adsorbed to the positive electrode and collected. With technical innovation, there is also a way to use negative electrode plates for dust collection. A greatest disadvantage of electrostatic dedusting method is that due to the small size of the gap between electrode plates, the gap is easily blocked by the dusts deposited on the electrode plate. Especially when the particles have a high viscosity, it is more likely that they block the gap between electrode plates and are difficult to be removed, so that the electrostatic dedustors lose their dedusting effects with an increased resistance. However, when the gap between electrode plates enlarges, a higher voltage is required for effecting dust removal, otherwise there is no dedusting capability. A higher plate voltage is likely to cause serious safety accidents. And a high manufacturing requirement for equipment exists with complex structures, thus manufacturing costs rise significantly. Moreover, the electrostatic dedusting method is only applicable to the dedusting for charged particles, and there is a certain range of application for the dust concentration of the gas to be purified.
In cyclone dedusting method, a dust-containing gas is subjected to rotary motion in a cyclone dedustor, creating centrifugal force to separate dusts from the gas, thus the separation effect is achieved. Cyclone dedusting method is characterized by simple equipment and low costs; but there is a disadvantage of poor separation effect that only large dust particles can be separated, leaving no effect for tiny particles.
In the bag-type dedusting method, a bag is fixed, such that a dust-containing gas flows from outside the bag to inside thereof. When the dust-containing gas is passing through the outer surface of the bag, dusts will be blocked outside by the bag fibers, while the gas is allowed to enter the bag, and dusts in the gas are removed in this way. The bag-type dedusting method is characterized by simple equipment, low costs and simple operations. Its disadvantage is yet very significant that, when the dusts are strongly adhesive, the bag is very likely to become caked and blocked, causing a loss of filtration effect and a sharply increased resistance, and production cannot be carried out. As the bag-type dedusting method relies on gas filtration by fiber bag for dust removal, its filtration effect is thus dependent on the pore size of the bag fibers. However, the smaller the pore size is, the greater will the resistance be. Dust particles larger than the pore size of the bag fibers can be removed, while those smaller than the pore size of the bag fibers will pass through the bag with the gas and cannot be removed.
In the water-dampness type dedusting method, water is sprayed directly into a dust-containing gas stream, and water-wettable particles in the gas can be agglomerated into large particulates by water and precipitate, so that the dusts are removed. With the water-dampness type dedusting method, various hydrophilic particles in the gas can be removed, whereas lipophilic particles cannot be removed. Although its dedusting effect is better than that of bag-type dedusting, as water has a boiling point of mere 100° C. and vaporizes easily, the vaporized water will be taken away by the gas. When the gas temperature is high, water loss is serious. The water-dampness type dedusting method is especially not suitable for water-deficient areas. As wet dedusting requires a consumption of a lot of water, various industrial alkaline wastewaters are usually taken as dedusting agents in practical use, but a higher requirement for dust-containing wastewater recycling and processing is also raised. Meanwhile, conventional methods of flue gas dedusting are incapable of removing dioxins, polycyclic aromatic hydrocarbons, other organic substances and heavy metal compounds in gases.